Esteban Bullrich, formerly Macri's education minister, received 33.9 percent of voter intention in the contest for Buenos Aires province, home to about 40 percent of voters, according to a poll by consultancy Management & Fit seen by Reuters. Fernandez earned 29.6 percent of voter intention.

Another poll by Poliarquia Consultores published in La Nacion newspaper showed Bullrich earning 37.7 percent of voter intention, compared to 35.2 percent for second-place Fernandez. A loss would diminish her chances of returning to the presidency in 2019.